The BSA A7-A10 Forum

Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: Atoll on 27.07. 2018 12:22

Title: Tacho drive
Post by: Atoll on 27.07. 2018 12:22

Now that it seems I have sourced a timing cover with the provision for a tacho drive what would be the best tacho drive gearbox to purchase and gear ratio for an A7SS 1958? They seem very expensive for what they are. 

Title: Re: Tacho drive
Post by: bsa-bill on 27.07. 2018 13:12

Available yes I think so possible from guys here, you also need the drive nut for the magneto.
I'm not familiar with the ins and outs (ratio)but guys here will be, possible depends on the rev counter you fit

Title: Re: Tacho drive
Post by: chaterlea25 on 27.07. 2018 13:31

Hi Atoll,
You will need a 2:1 drive box, the rev counter will then be 4:1 ratio
Gold Stars used a 1:1 ratio drive which "looks" the same
The problem with the goldie setup is that with the 2 x higher cable speed and moving parts in the rev counter they wear out rapidly
The drive boxes are available here http://www.chronometrics.co.uk/html/drives.html

Hopefully your A7 has a manual advance mag? the rev counter won't work with an auto advance type

John

Title: Re: Tacho drive
Post by: muskrat on 27.07. 2018 21:42

G'day Atoll.
As John said only for manual advance magneto. NOS one in the US https://tinyurl.com/y8kp64c8 the extended nut from Draganfly https://tinyurl.com/y9dgm2yk I use an alloy pinion https://tinyurl.com/y9kem6tf
Or you can just blank the hole off.
Cheers

Title: Re: Tacho drive
Post by: JulianS on 28.07. 2018 09:48

George Prew, A10 and Goldie parts supplier used to advertise a rev counter drive nut which fits auto advance unit. If you want such it would be worth a call to see if it is still available.

Title: Re: Tacho drive
Post by: Atoll on 28.07. 2018 10:10

Thanks for all the help guys much appreciated.

Title: Re: Tacho drive
Post by: chaterlea25 on 28.07. 2018 20:22

Hi All,
I could be wrong but my readings would suggest that the drag from the rev counter drive prevents the auto advance from working  (advancing) *????*

John

Title: Re: Tacho drive
Post by: muskrat on 28.07. 2018 21:01

May be right John, but would need a special nut made up.
Hay Atoll is it a manual or auto mag? SS should have a manual.
Cheers

Title: Re: Tacho drive
Post by: trevinoz on 29.07. 2018 22:48

John, are you sure that the A10 used a 4:1 tacho?
I have two with gearboxes which are 2:1 instrument and 1:1 gearbox.

Title: Re: Tacho drive
Post by: chaterlea25 on 29.07. 2018 23:44

Hi Trevor,
I could be wrong (easily *sad2*)
I'm sure there was a previous thread with a link to the original spec's
I had it on my old PC but not on the current laptop
I found a NOS 2:1 drive and will fake a 4:1 rev counter face to look right for my B33.9 project  *eek*

John

Title: Re: Tacho drive
Post by: JulianS on 30.07. 2018 09:09

The gearboxes were 2:1 reversing

The tacho - usually face number RC83 - were also 2:1. With up to 8000 rpm.

Hgher rev range tachos are listed as 4:1

From 1959 Smiths catalogue.

Title: Re: Tacho drive
Post by: Atoll on 30.07. 2018 10:44

Quote from: JulianS on 30.07. 2018 09:09

The gearboxes were 2:1 reversing

The tacho - usually face number RC83 - were also 2:1. With up to 8000 rpm.

Hgher rev range tachos are listed as 4:1

From 1959 Smiths catalogue.

Now you guys are confusing me  *smiley4*

Title: Re: Tacho drive
Post by: trevinoz on 30.07. 2018 22:20

Julian, the tacho is 2:1 which needs a 1:1 gearbox as I have.
My tacho is RC 83.